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京 都 大 学 大 学 院 理 学 研 究 科

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要旨集

２０１３年１月２１日（月）物 理 学 第 一 分 野

物理学第一分野ＤＣ３回生研究発表会
場所：理学研究科５号館 ５階・第四講義室
発表：１５分（別に質問１０分程度）
２０１３年１月２１日（月）９：００～

開始

目

次

１．Steplike electric conduction in a classicaltwo-dimensional electron system through a narrow constriction in a microchannel
荒木
元（ ９：００）・・・・・

１

２．Nonequilibrium Dynamics of Deformable Self-Propelled Particles
市野悠（９：２５）・・・・・

２

３．Non-centrosymmetric superconductivity in d-electron compounds江口学（９：５０）・・・・・

３

４．Visualization of the Spatial Distribution of Order Parameter inSuperfluid 3He with Nuclear Magnetic Resonance Imaging
金本 真知（１０：１５）・・・・・

４

１０：４０～１０：５０

休憩

５．Novel ferromagnetic quantum criticality in the heavy-fermion iron oxypnictide Ce(Ru1-xFex)PO
北川 俊作（１０：５０）・・・・・ ５
６．Vacancy trapping and spin-flip transitions of paraexcitons incuprous oxide studied by visible and terahertz spectroscopy
Sandhaya Koirala（１１：１５）・・・・・

６

７．Controlling the Dynamics of a Localized Domain in an Excitable
Reaction-Diffusion System by a Time-Delayed Feedback設樂 恭平（１１：４０）・・・・・

７

８．Nonlinear Transport Phenomena in Semiconductor
Studied by Terahertz Nonlinear Spectroscopy
篠北

１２：３０～１３：３０

啓介（１２：０５）・・・・・

８

昼休み

９．An SU(6) Mott insulator of an atomic Fermi gas realized by large-spin Pomeranchuk cooling
田家慎太郎（１３：３０）・・・・・ ９
１０．Mesoscopic Heterogeneity and large response to external fields in Glassy Ferroelectric Crystals
高江 恭平（１３：５５）・・・・・１０１１．Ultrafast Carrier Dynamics in Graphene under a High Electric Field
谷 峻太郎（１４：２０）・・・・・１１
１２．Resistance Enhancement in the ν = 2/3 Quantum Hall State
津田 是文（１４：４５）・・・・・１２

１５：１０～１５：２０

休憩

１３．Novel mesophase in the region between Cholesteric and Perforated Lamellar -Nematic phases吉岡潤（１５：２０）・・・・・１３

Steplike electric conduction in a classicaltwo-dimensional electron system through a narrow
constriction in a microchannel
Advanced statistical dynamics group

Moto Araki

Abstract Using molecular dynamics simulation, we investigate transport properties of a classicaltwo-dimensional electron system confined in a microchannel with a narrow constriction. The calculated
conductance in the simulations exhibits steplike increases as reported in a recent experiment [1].
© 2013 Department of Physics, Kyoto University

Using molecular dynamics simulations, we investigate transport properties of classical two-dimensional
electrons through a microchannel with a narrow constriction. The electrons are confined by the electric potential
derived from the Poisson equation under boundary conditions to imitate a device structure in an experiment, andinteract with each other. The electron system of our simulation is a realistic model for strongly correlated
electrons on liquid helium in the microscale device. In the simulations, the electrons are driven by the chemical
potential difference between two particle baths.[2]As a function of the confinement strength of the constriction, the calculated conductance in the simulations
exhibits steplike increases as reported in a recent experiment [1]. It is confirmed that the number of the steps
corresponds to the number of stream lines of electrons through the constriction. For the strong confinement, theconductance is affected by the temporal fluctuation of electrostatic potential which induces intermittent
disappearances of a potential barrier in the constriction. We also observe the temporal change of the number ofelectrons’ streams through the constriction because of the short-wavelength density fluctuations [2]. Therefore, we
suppose that the stream lines forced by the confinement cause the steplike electric conduction, and the fluctuation
plays a certain role in smoothing the steps.

Simulation

2 .0

1 .0

0 .0

2 .0

G E ( MΩ -1)−1

G (M Ω )

3 .0

0.00

0.20
0.10
0.30
Gate Voltage VG (V)Experiment

1 .0

0.0
0.00

0.50
1.00
Gate Voltage VG (V)

1.50

Fig. 1. The calculated conductance G in the simulations (left figure) and the measured conductance GE in theexperiment [1] (right figure) versus the gate voltage VG. As VG is lower, the confinement of the constriction is stronger.

References[1] D. G. Rees et al., Phys. Rev. Lett. 106, 026803 (2011).
[2] M. Araki and H. Hayakawa, Phys. Rev. B 86, 165412 (2012).

Nonequilibrium Dynamics ofDeformable Self-Propelled Particles
Nonlinear Dynamics GroupYu Itino

Abstract We present a theoretical study on assemblies of deformable self-propelled
particles (SPP). Starting from the single particle model of a deformable SPP (Ohta-Ohkuma
model), we introduced the short-ranged repulsive force between those particles. The system
exhibits ordered, disordered, and coexistence state by changing the density and the noisec 2013 Department of Physics, Kyoto University
strength.
The spontaneous order of the self-propelled particles (SPP) has been studied over a decade. So
called “Vicsek model” [1] and its variants exhibit collective polar order due to the explicit alignment
rule between particles. The breakdown of the collective motion under strong external noise has been amain concern of current studies.
In addition to minimal variables of SPP (position and velocity), our deformable SPP has a nematic
deformation tensor coupled with the polar velocity [2]. We introduced the short-ranged repulsive interaction between particles, and instead of explicit alignment rule, the deformation-dependent magnitude
was included to the multi-particle model [3, 4]. The following figure shows the emergence of the collective (ordered) motion starting from (a) random initial state, followed by (b) intermediate ordered region,and (c) globally ordered structure.

The (polar) order of the system can be destroyed by external randomness. We studied the orderdisorder transition induced by increasing density and the noise. The transition is confirmed to be discontinuous, and the hysteresis has been observed. The nature of the transition is qualitatively explainedby the mean field theory.
Compared to the diffusive disordered state, the ordered state contains much complex and defective
structure. The collective swirling motion within the ordered region has been observed. Furthermore,
a novel band structure within the disordered domain, invading ordered region, has been found [5]. To
quantitatively address those phenomena, we developed a real-space clustering method to classify ordereddomains.
References[1] T. Vicsek, et. al., Phys. Rev. Lett. 75 1226 (1995).
[2] T. Ohta and T. Ohkuma: Phys. Rev. Lett. 102 154101 (2009).
[3] Y. Itino, T. Ohkuma, and T. Ohta, J. Phys. Soc. Jpn. 80 033001 (2011).
[4] Y. Itino, and T. Ohta, J. Phys. Soc. Jpn. 81 104007 (2012).
[5] M. Tarama, Y. Itino, A. M. Menzel, and T. Ohta, EPJ (to be published).

Non-centrosymmetric superconductivity
in d-electron compounds
Quantum Materials Laboratory	  	  Gaku EGUCHI
Abstract Superconductivity in the absence of crystalline inversion symmetry in d-electroncompounds is studied. We succeeded in growing polycrystalline CaMSi3 (M=Ir, Pt) andsingle-crystalline CaIrSi3, and revealed their superconducting behavior. We also studied thenovel spin singlet-triplet mixed superconducting state in Li2(Pd1-xPtx)3B.
© 2013 Department of Physics, Kyoto University

Superconductivity in the absence of inversion symmetry has been attracting much attention because of
its potential exotic superconducting phenomena. In bulk materials, such superconductivity is realized in crystals
without inversion centers; these superconductors are nowadays called the non-centrosymmetricsuperconductors[1]. The absence of inversion symmetry results in two important features. First, thesuperconducting state can no longer be classified either as a spin-singlet or a spin-triplet state, but is asinglet-triplet mixed state. Second, the electronic energy bands exhibit a momentum-dependent effective Zeeman
splitting due to the antisymmetric spin-orbit interaction.
We studied some d-electron-based non-magnetic non-centrosymmetric superconductors. These
compounds should be appropriate to examine the novel pairing state because novel features specific tonon-centrosymmetric superconductivity can be extracted without other physics such as strong electron
correlations and magnetism.
We succeeded in growing polycrystalline CaMSi3 (M=Ir, Pt) (Fig. 1, left) and reveled their fully-gappedsuperconducting nature and the dominance of electron-phonon scattering[2,3]. What is more, we also succeededin growing single-crystalline CaIrSi3 and revealed the presence of strong spin-orbit interaction and weakly
anisotropic superconducting nature[4]. We also investigated physical properties of polycrystalline Li2(Pd1-xPtx)3B(Fig.1, right) systematically, and revealed a phonon anomaly as well as anisotropic superconducting gaps in
Pt-rich members, including their magnetic field dependence[5].
Among these works done in my Ph. D course, I focus on the single-crystalline CaIrSi3 for thispresentation. I explain single-crystal growth of CaIrSi3 and its normal and superconducting properties, and thendiscuss the possibility of a novel spin singlet-triplet mixed state.
	
 

	
 
	
 

Fig. 1. Crystal structures and their inversion of non-centrosymmetric CaMSi3 (M=Ir, Pt) andLi2(Pd1-xPtx)3B. The figures are drawn by VESTA[6].

References[1] E. Bauer et al., Phys. Rev. Lett. 92, 027003 (2004).
[2] G. Eguchi et al., Phys. Rev. B 83, 024512 (2011).
[3] G. Eguchi et al., J. Phys. Soc. Jpn. 81, 074711 (2012).
[4] G. Eguchi et al., Phys. Rev. B 86, 184510 (2012).
[5] D. C. Peets, G. Eguchi et al., Phys. Rev. B 81, 054521 (2011), etc.
[6] K. Momma and F. Izumi, J. Appl. Crystallogr. 44, 1272 (2011).

Visualization of the Spatial Distribution of Order Parameter inSuperfluid 3He with Nuclear Magnetic Resonance Imaging
Low Temperature Physics Laboratory

Masatomo Kanemoto

Abstract Resonance frequency of superfluid 3He in NMR depends on the local orientation of internal
degree of freedom of the order parameter. I developed the method for visualizing the spatial distributionof resonance frequency in NMR in order to study inhomogeneous order parameter in superfluid 3He.
© 2013 Department of Physics, Kyoto University

Superfluid 3He is a p-wave pair condensate with multiple internal degrees of freedom. So superfluid 3He has somephases. In the A-phase, the order parameter is expressed by two vectors, d and l. The spatial distribution of the orderparameter is called as the texture. The texture is determined by minimizing the total energy, which depends on
applied magnetic fields and the shape of the container wall. NMR signals in A-phase are characterized by afrequency shift from Larmor frequency which is determined by applied magnetic field. The frequency shift depends
on the distribution of the order parameter. In standard NMR, the signal is integrated over the space in the container.
No information on the location of signal source can be obtained. NMR spectrum under various magnetic field
gradient direction provide MRI image, which is a spatial distribution of signal source. However in the standard MRI,
spectroscopic information is destroyed. In order to measure the distribution of the frequency shift in space, I
developed frequency-resolved MRI method.
A parallel plate cell whose gap is 100 microns is constructed. Static magnetic field is applied parallel to the plate. In
A-phase, l orients perpendicular to the wall of the container, while d tends to beperpendicular to the magnetic field. The d and l orients parallel or anti-parallelwith each other due to dipole interaction. A homogeneous texture, in which dand l are perpendicular to the plates, satisfies all these conditions. If there areparallel and anti-parallel domains, where d and l orient parallel or anti-parallelwith each other, there appears a region where d and/or l rotate gradually inspace in order to connect domains on each side. This region is called thedomain wall. Such a domain wall was observed by NMR measurement
previously. However nobody has ever studied why this energeticallyunfavorable state can be stabilized. The MRI measurement of the topologicalobjects in superfluid 3He, such as the domain wall, opens the doorway toFig. 1further understand topological superfluid.
8
Intensity (a.u.)

To demonstrate the frequency-resolved MRI, spatial variation of the static
magnetic field strength in the parallel plate cell is measured (Fig.1). The
bright circle corresponds to the sensitive region of the NMR coil in the plate.
Gradations inside the circle indicate the strength of local magnetic field in
57Hz step at the Larmor frequency of 5.4MHz. The NMR spectrum shows thecorresponding lineshape with long tail on low frequency side (Fig.2). Saddle
shaped distribution of the static magnetic field in Fig.1 explains the lineshapein Fig.2 very well. This indicates the feasibility of the frequency-resolved
MRI as advanced NMR spectroscopic measurement.

6
4
2
0
-0.5

0
0.5
Frequency[kHz]Fig. 2

1

Novel ferromagnetic quantum criticality in the heavy-fermion
iron oxypnictide Ce(Ru1-xFex)PO
Quantum Materials Lab.

Shunsaku Kitagawa

Abstract We have performed 31P-NMR measurements on Ce(Ru1-xFex)PO in order to investigate
ferromagnetic (FM) quantum criticality. Our NMR results suggest that the FM quantum critical point at x
∼ 0.86 is induced by the tuning of the magnetic dimensionality, different mechanism from that believedin heavy-fermion compounds.
© 2013 Department of Physics, Kyoto University

Quantum phase transition in itinerant magnets is one of the major topics in a strongly correlated electron system.
Up to now, there have been considerable efforts for understanding the nature of a QCP on various
antiferromagnetic heavy fermion (HF) compounds, but very few on ferromagnetic (FM) HF compounds,
particularly on Ce-based FM HF compounds. The iron oxypnictide CeFe(Ru)PO is a related material of theiron-based superconductor LaFePO. They possess the same 2D layered structure, stacking the Ce(La)O andFe(Ru)P layers alternatively. The CeO layer contributes to the large magnetic response, and the Fe(Ru)P layer isconductive in CeFe(Ru)PO. CeRuPO is a FM HF system with Curie temperature TCurie = 15 K and coherent
temperature TK ~ 10 K[1]. On the other hand, counterpart CeFePO is a HF compound with a PM ground state
down to 80 mK and TK ~ 10 K[2,3]. Therefore, we expect that the ground state can be tuned continuously fromFM to PM state by substituting isovalent Fe for Ru.
We performed 31P-NMR on Ce(Ru1-xFex)PO to investigate the evolution of magnetic properties at low
temperatures. NMR measurements can probe static and dynamic magnetic properties. Knight shift K detects static
spin susceptibility at q = 0, while nuclear spin-lattice relaxation rate 1/T1 probes q-summed spin fluctuations
perpendicular to the applied magnetic field. TCurie and the internal field at the P site, proportional to orderedmagnetic moment ‹ord›, continuously approach zero toward x ~ 0.86 by Fe substitution for Ru, suggestive of thepresence of FM QCP at x ~ 0.86 (Figure) [4], which is in sharp contrast with FM criticality observed in other FM
compounds. For example, FM transition in UGe2 is gradually suppressed by applying pressure, but the transitionchanges from second order to first order at a tricritical point and the first order metamagnetic transition emerges ina small external field[5].
In addition, we discussed the variation of magnetic-fluctuation character in Ce(Ru1-xFex)PO from the relationshipbetween K and 1/T1. Our experimental results suggest that
the FM QCP in Ce(Ru1-xFex)PO is induced by the tuning of
the dimensionality of the magnetic correlations from three
dimensionality to two dimensionality. This mechanism isquite different from that believed in HF compounds anditinerant FM compounds, rather similar to the dimensionaltuning realized in superlattice CeIn3/LaIn3[6]. The phase
diagram difference between Ce(Ru1-xFex)PO and the otherFM systems may originate from the difference of the tuning
parameter.
References[1] C. Krellner et al., Phys. Rev. B 76, 104418 (2007).
[2] E. Brüning. et al. Phys. Rev. Lett. 101, 117206 (2008).
Figure: The T–x–H phase diagram on[3] S. Kitagawa et al., Phys. Rev. Lett. 107, 277002 (2011).
Ce(Ru1-xFex)PO obtained by 31P-NMR. The[4] S. Kitagawa et al., Phys. Rev. Lett. 109, 227004 (2012).
inset shows x dependence of ordered magnetic[5] H. Kotegawa et al., J. Phys. Soc. Jpn. 80, 083703 (2011).
moment ‹ord›at 0H ≈ 0.5 T.
[6] H. Shishido et al., Science 327, 980-983 (2010).

Vacancy trapping and spin-flip transitions of paraexcitons incuprous oxide studied by visible and terahertz spectroscopy
Solid State Spectroscopy Group

Sandhaya Koirala

Abstract We have investigated the trapping mechanism of free excitons into crystal defects in a semiconductor
cuprous oxide. By using high grade samples containing small amounts of oxygen vacancies, we find the direct
correlation between the lifetimes of free paraexcitons and trapped paraexcitons. In addition, we have designed
terahertz time-domain spectroscopic measurements for the detection of paraexcitons.
© 2013 Department of Physics, Kyoto University
An ensemble of excitons or bound electron-hole pairs in a semiconductor is predicted to undergo a transition
into a Bose-Einstein condensation (BEC) phase at low temperatures. Due to the long lifetime, 1s paraexcitons incuprous oxide (Cu2O) are believed to be a good candidate to realize excitonic BEC in a thermal equilibrium [1].
To investigate excitonic BEC states, two parameters are very important; one is the exciton lifetime and the other isthe exciton density. Since the lifetime of the 1s paraexcitons in Cu2O is limited by some unknown non-radiative
recombination processes, we clarified the lifetime shortening mechanism by visible spectroscopy. In addition, we
purpose a novel technique of monitoring the paraexciton density by using spin-flip transitions in the terahertz
region.
We performed time-resolved spectroscopic measurements by taking natural crystals of cuprous oxide
containing various amounts of oxygen vacancy. Based on the temperature dependence of the photoluminescence
spectra, we obtained the activation energy of EA=33 meV for trapping of free excitons into oxygen vacancy. We
found the correlation between lifetimes of free and trapped excitons (Fig.1a), and established four-level energy
model to explain the temperature dependent lifetimes (Fig.1b). From the comparative studies of four different
samples, it is clear that the existence of the oxygen vacancy is crucial in limiting the free-exciton lifetime inhigh-quality cuprous oxide [2].
So far, excitonic Lyman spectroscopy has been performed to determine the 1s paraexciton density [3]. Thismethod induces a transition from 1s to 2p paraexcitons separated by 129 meV, accompanying emission of manyphonons and possible unwanted heating. Here, we propose to use the recent terahertz technology to monitor the
spin-flip transition of 1s paraexcitons, from the triplet to the singlet states separated by 12 meV. We calculated
selection rules for these spin-flip
transitions under external magnetic
fields. Using a strong terahertz
generation system which werecently build and checked theperformance, we will perform
optical-pump and terahertz-probe
experiments. Our method is
expected to be useful in detecting
an excitonic BEC state in a
potential well created by stressunder a strong magnetic field at Figure 1: (a) Lifetimes of free and trapped excitons as a function of temperature.
low temperatures [4].
Simulated lifetimes are shown by dashed lines. (b) Schematic diagram of the
four-level scheme used for the simulation.

Reference:[1] K. Yoshioka et al., Nature Commun. 2 (2011) 328.
[2] S. Koirala et al., Journal of Lumin 134 (2013) 524.
[3] T. Tayagaki et al., J. Phys. Soc. Jpn.74 (2005) 1423.
[4] Ch. Standford et al., Phys. Rev. B 84 (2011) 165215.

Controlling the Dynamics of a Localized Domain in an Excitable
Reaction-Diffusion System by a Time-Delayed Feedback
Nonlinear Dynamics Group Kyohei Shitara
Abstract We introduce a time-delayed feedback to reaction-diffusion equations having a solution of a
localized domain. As the result of it, it is found that we can make a motionless domain traveling and a
traveling domain motionless by changing a sign of the feedback strength. In addition to it, a variety ofother behaviors are also found.
© 2013 Department of Physics, Kyoto University

A reaction-diffusion system is one of dissipative systems and exhibits a variety of pattern formation phenomena.
To reveal the dynamics of these patters has great potential for applications as well as contributions to an
investigation of non-equilibrium systems. Many scientists try to control the dynamics of these patterns.
A localized domain is one of patterns in reaction-diffusion systems. It is known that there is a drift bifurcation
where a motionless domain becomes unstable and begins to travel [1, 2]. However, whether its bifurcation can berealized or not in experiments depends on systems. Therefore, we have tried to control a motion of a localized
domain.
Tlidi and his co-workers have shown that a motionless cavity soliton can be driven by a time-delayed feedback[3]. Following their method, we have introduced a time-delayed feedback to reaction-diffusion equations having
excitability in one dimension as shown below.

∂u 2 ∂ 2 u=ε+ f (u) − v + Λ[u(t − T) − u(t)]
∂t
∂x 2
.
∂v
∂ 2v
= D 2 + u − γv
∂t
∂x

τε

where Λ and T are the strength of the delayed feedback and the delay time, respectively. We have found that when
the sign of Λ is negative, a motionless domain becomes unstable and begins to travel as shown in Fig. 1(a). On theother hand, when the sign of Λ is positive, a traveling domain becomes unstable and motionless as shown in Fig.
1(b). Moreover, a variety of other behaviors of the domain are also found such as traveling with oscillation,
rocking, and creation-annihilation.

Fig. 1. (a) A motionless domain begin to travel at t=10 when Λ is negative. (b) A traveling domain become
motionless at t=10 when Λ is positive.
References[1] T. Ohta, Physica D, 151, 61 (2001).
[2] K. Krischer, A. Mikhailov, Phys. Rev. Lett. 73, 3165 (1994).
[3] M. Tlidi, A. G. Vladimirov, D. Pieroux, and D. Turaev, Phys. Rev. Lett. 103, 103904 (2009)Nonlinear Transport Phenomena in Semiconductor
Studied by Terahertz Nonlinear Spectroscopy
Solid State Spectroscopy Group

Keisuke Shinokita
Abstract I will present that the intense THz pulses induce strong spectral modulations in excitonic and band-edge
absorption of GaAs/AlGaAs multiple quantum wells. Furthermore, it is introduced that the 1-MV/cm electric fieldof a THz pulse allows us to excite electrons from valence to conduction band of GaAs/AlGaAs MQW and observebright exciton luminescence.
© 2013 Department of Physics, Kyoto University

The nonlinear interaction of matter with strong, oscillating electric fields has attracted considerable attention
because of their importance in fundamental physics and technological applications. The resonant (or nearlyresonant) interaction of electric fields with electronic transitions in semiconductors and atomic gases exhibits
intriguing nonlinear phenomena such as the ac (optical) Stark effect, Rabi oscillations, Autler-Townes splitting,and electromagnetically induced transparency. Even in nonresonant situations, atomic and condensed mattersystems subjected to high-intensity laser excitation exhibit extreme nonlinear phenomena such as above-thresholdionization, high-order harmonic generation and various nonlinear transport phenomena. These phenomena cannot
be described by perturbation theory. The recent developments in high-power table-top terahertz (THz) pulse
sources enable us to coherently drive low-energy transitions into the nonlinear regime and to study fascinating
nonlinear transport phenomena in various materials [1].
In this work we studied the nonlinear interaction of excitons and bandgap in GaAs/AlGaAs multiple quantum
wells (QWs) by using THz-pump (below 0.1 MV/cm) and optical-probe spectroscopy [2]. This technique revealed
that the intense single-cycle THz pulses induce strong spectral modulations in the excitonic and band-edge
absorption of QWs. Furthermore, by using the ultra-intense THz pulse (a 1 MV/cm electric field it is indicatedthat it allows us to excite electrons from the valence band to the conduction band of the QWs and observe exciton
luminescence [3]. To obtain the influences of trap states on the nonlinear transport phenomena under
instantaneous high-electric fields, we studied the photoexcited carrier dynamics in cleaner QWs by using
time-resolved photoluminescence spectroscopy involving the simultaneous use of continuous-wave visible laser
and intense THz pulse excitations [4]. It is found that the intense THz pulse induces the photoluminescence (PL)flash from the undoped QWs under continuous wave laser excitation, and the number of excitons increases byfour orders of magnitude compared with the steady-state exciton density. Furthermore, the absence ofTHz-induced exciton generation in this study suggests that in high-quality GaAs wells, the preservation of thek-conservation rule because of lack of impurity or defect states significantly suppresses the impact ionization rate.
References[1] H. Hirori, A. Doi, F. Blanchard, and K. Tanaka, Appl. Phys. Lett. 98, 091106 (2011).
[2] K. Shinokita, H. Hirori, M. Nagai, N. Satoh, Y. Kadoya, and K. Tanaka, Appl. Phys. Lett. 97, 211902 (2010).
[3] H. Hirori, K. Shinokita, M. Shirai, S. Tani, Y. Kadoya, and K. Tanaka, Nature Commun. 2, 594 (2011).
[4] K. Shinokita, H. Hirori, K. Tanaka, T. Mochizuki, C. Kim, H. Akiyama, L. N. Pfeiffer, and K. W. West, submitted for publication.

An SU(6) Mott insulator of an atomic Fermi gas realized bylarge-spin Pomeranchuk cooling
Quantum Optics Group

Shintaro Taie

Abstract We report the realization of a Mott insulator of an atomic Fermi gas with novel SU(6) spinsymmetry. Besides several characteristics of a Mott phase such as vanishing compressibility, we find thestrong adiabatic cooling effect which originates from its large spin degree of freedom.
© 2013 Department of Physics, Kyoto University

The SU(N) Hubbard model, which is obtained by extending SU(2) spin symmetry of the usual Hubbard model toSU(N>2), has attracted much theoretical interests in the context of searching for the ground states without
long-range order, such as spin liquids [1].
We report the first experimental realization of a Mott insulator with enlarged SU(N) spin symmetry, with an atomic
Fermi gas of 173Yb in a three-dimensional optical lattice [2]. 173Yb has an SU(6) symmetric interaction with respect
to its nuclear spin I=5/2 and realizes the SU(6) Hubbard model in the optical lattice. First, we observe several
features that is common to Mott insulating phases. By using photoassociation technique, we measure double
occupancy in the lattice, and observe the significant reduction of double occupancy in the strongly interacting
regime. From this behavior of double occupancy, we extract the compressibility of the system and find it consistent
with the emergence of a Mott insulating core with unit filling in the trap center. We also confirm the existence of thecharge excitation gap on the order of on-site interaction U, by applying periodic amplitude modulation of the latticepotential.
Lattice modulation spectroscopy can also provide information of nearest-neighbor correlations of atoms in opticallattices [3]. We apply thermometry based on this correlation measurement to both SU(6) and SU(2) systems under
the same experimental conditions, and find that the SU(6) system has much lower temperature compared with theSU(2) system (Fig.1). This remarkable result can be attributed to the enhancement of adiabatic cooling during theloading process into the lattice, owing to the large spin degree of freedom in the localized Mott phase. Thismechanism is analogous to Pomeranchuk cooling in solid 3He and advantageous for realizing exotic quantum
phases of the SU(N) Hubbard model at extremely low temperature.

Fig. 1. (a) Measured temperatures of SU(6) (closed circles) and SU(2) (open circles) Mott insulators of 173Yb
in a 3D optical lattice. (b) Expected density and entropy profiles for the data with lowest temperatures in (a).

References[1] For example, N. Read and S. Sachdev, Nucl. Phys. B 316, 609-640 (1989)[2] S. Taie, R. Yamazaki, S. Sugawa and Y. Takahashi, Nature Phys. 8, 825–830 (2012).
[3] D. Greif, L. Tarruell, T. Uehlinger, R. Jördens and T. Esslinger, Phys. Rev. Lett. 106, 145302 (2011)

Mesoscopic Heterogeneity and large response to external fields in
Glassy Ferroelectric Crystals
Phase Transition Dynamics GroupKyohei Takae

Abstract We construct a molecular dynamics model of glassy ferroelectric crystals, which consist of
spheroids possessing dipolar moments and non-polar isotropic impurities. Mesoscopically ordereddomains are conspicuous. Large dielectric and piezoelectric responses with hysteresis loops are also
marked under applied electric fields at low temperatures.
© 2013 Department of Physics, Kyoto University

Certain crystals composed of anisotropic particles undergo orientation phase transitions at low temperature withspontaneous lattice deformations. With addition of impurities, however, the macroscopic order does not grow at
low temperatures, yielding glassy crystalline states [1,2]. In real substances including ferroelectric relaxors,
furthermore, molecules often possesses dipolar moments, resulting in complex dielectric interactions. For example,
nanoscale freezing of ferroelectric domain (polar nanoregions) without ferroelectric phase transitions, and large
dielectric constants with characteristic frequency dependence, have been reported [3]. Large piezoelectric
responses are also conspicuous in some relaxors [4].
We here present molecular dynamics results of dipolar glass formation in crystals composed of polar spheroids
and non-polar isotropic impurities. In these glasses without macroscopic order, heterogeneous distribution ofmesoscopically ordered domains are conspicuous, as displayed in the left panel of Fig.1. Under applied electricfields, the glasses exhibit large polarisation responses with hysteresis loops at low temperatures, whereas
paraelectric response is observed at high temperatures. This characteristic behaviour of the dipole glasses isdisplayed in the middle panel and is analogous to ferroelectric materials. Furthermore, since the shape of dipolar
particles are anisotropic in our model, changes of the direction of dipolar moments lead to large piezoelectricstrain in the glassy ferroelectric crystals, as shown in the right panel of Fig.1.

Fig.1. Orientationally ordered domains in a glassy ferroelectric crystal at low temperature T (left). Polarisationand strain response to applied electric field (middle and right). Φ represents the applied voltage.

References[1] U. T. Höchli, K. Knorr, and A. Loidl, Adv. Phys. 39, 405-615 (1990).
[2] K. Takae and A. Onuki, EPL 100, 16006 (2012).
[3] L. E. Cross, Ferroelectrics 76, 241-267 (1987).
[4] S.-E. Park and T. R. Shrout, J. Appl. Phys. 82, 1804-1811 (1997).

Ultrafast Carrier Dynamics in Graphene under a HighElectric Field
Solid state spectroscopy group

Shuntaro Tani

Abstract Time-resolved high-field carrier transport in graphene is studied using terahertz-pump opticalprobe technique. The experimental results show good agreement with our numerical results, which
demonstrate the importance of efficient carrier multiplication by impact ionization.
© 2013 Department of Physics, Kyoto University

Graphene is a mono-atomic layer carbon material which possesses many unique properties arising from its
truly 2D honeycomb structure. Especially, its characteristic linear dispersion band structure makes electrons
massless, which obey the relativistic Dirac equation. Since an electron has a charge, the combination of light andgraphene provides us an ideal playground for the relativistic massless particles in solid state, where we can create
and manipulate the Dirac fermions, and monitor their dynamics using optical and terahertz (THz) techniques. One
major challenge is to understand the electron dynamics under a high electric field, which would establish thefoundation of the Dirac fermion transport with many body interaction. Since the momentum distribution ofelectrons is far out of equilibrium, many body interaction and various scattering processes play dominant roles.
This requires ultrafast time resolution for the measurement.
We have investigated the ultrafast carrier dynamics in graphene under high electric fields using 200 fs halfcycle THz excitation pulses and monitoring their dynamics with 50 fs duration near-infrared (NIR) probe pulses[1]. Right after the THz excitation, the transmission of graphene increases and returns back to its initial value
within 2 ps. This THz induced transparency is observed for the fist time in the NIR region, and should be causedby the electron filling or hole depletion of the corresponding energy levels, allowing us to monitor the timeresolved carrier dynamics under the THz field. Measurements with various THz field strength and quantitative
analysis show that the carrier acceleration by the intense THz field is far more efficient than that is expected fromthe previous models.
To account for our experimental results, we have calculated the distribution function of carriers in the
momentum space using the semiclassical Boltzmann equation. What is new in our calculation is that we haveincluded many body interband scattering processes, which changes the number of carriers (i.e., impact ionization
and Auger recombination). Although the occurrence of these processes has been controversial, the good
agreement between experimental and numerical results demonstrate the importance of efficient carrier
multiplication by the impact ionization in graphene.
Figures 1(a)–1(e) show calculated distribution of holes in the momentum space at various time delays for the
maximum applied electric field of 300 kV/cm. Figures 1(f)–1(j) show the corresponding angle-resolved energy
distribution of holes. Before THz pulse excitation, holes are distributed within Fermi energy with 300 K thermal
fluctuation [Figs. 1(a) and 1(f)]. As a THz electric field is applied, the carrier distribution becomes an asymmetricshape [Figs. 1(b) and 1(g)], and keeps its asymmetry until the peak electric field [Figs. 1(c) and 1(h)]. After theTHz excitation pulse is passing through the sample, the distribution immediately recovers its symmetric shape[Figs. 1(d) and 1(i)] and returns back to the initial distribution by emitting optical phonons [Figs. 1(e) and 1(j)].

c-c

ETHz

800 nmpulse

THz puls

IIe

EF < -200 meV

op

Valence band

Suppression of 800-nm absorptionT
ac Hz fce iel
ler d
ati
on

Conduction band

Population(a)(b)1.0To Lock-in amp.

Dirac cone at K-point

Graphene on SiO2
0.5

E
vF

0.6
0.4 Time
0.2 plate

0 fs (d)100 fs (e)

(h)

(i)

(j)

900 fsVariable
𝑬THz
delay

kx

(f)
0.8Ge plate- 50 fs (c)

𝜽

ky

0.0
1.0

0

- 100 fs (b)Photo diodef! (E)

(a)

(g)

!=0
! = "/2!="

T = 1760 K

T = 2860 K

T = 2570 K

T = 1610 K

800 nm
pulse

"-BBO

0.0
0

1000

Dual
!-BBO 500 HzwaveplateEnergy (meV)
chopper

0

1000
0
1000
0
1000
0
1000
Energy (meV)
Energy (meV)
Energy (meV)
Energy (meV)ETHz (kV/cm)

SpectrumFig. 1(a)–(e) Distribution functionof holes in the momentum space with various time delays. Dashed0(c) momenta corresponding
10
circles represent
to the energy of 800 nm transition. (f)–(j) Angle-resolved
energy distribution of holes. Fermi-Diracfitting is represented as a dashed line. Vertical arrows indicate
-3
200
10
the energy of 800 nm transition.
0

10

20

D / OD[1] S.Tani, F.Blanchard, and Frequency
K.Tanaka,(THz)Phys. Rev. Lett. 109, 166603 (2012)
0

(d)
0.10

Resistance Enhancement in the ν = 2/3 Quantum Hall State
Nano-scale Quantum Condensed Matter Physics Laboratory

Shibun Tsuda

Abstract We have investigated the quantum Hall state where longitudinal resistance Rxx is enhanced with nuclear spin polarization induced by large current at the Landau level filling factor ν= 2/3. With the measurement of temperature dependence and pump current frequency dependence of Rxx, we find out that the resistance enhanced state is an insulator and is effectively enhanced by AC current.
The spin degree of freedom plays an important role in the fractional quantum Hall States (QHSs). It is responsible for a variety of phenomena such as phase transitions between different ground states and interactions of theelectronic system with the nuclei. At the spin phase transition point in ν = 2/3 QHS, an anomalous longitudinal
resistance Rxx peak is observed. The enhancement of the Rxx is explained by a domain structure of the two different
spin phases of ν = 2/3 QHS [1]. It is believed that when current pass across a domain boundary, electron spins
flip-flop scatter nuclear spins causing dynamic nuclear spin polarization (DNP), then the spin polarization aﬀects
back the domain formation and increases the length of the domain boundaries. The nuclear spin relaxation after
DNP in the ν = 2/3 QHS is applied to study of various QHSs [2]. However, details of the domain structure and themechanism of the Rxx enhancement are still unclear. Previous experiment couldn’t suﬃciently study the resistanceenhanced state (RES) because the RES is not stable and decays over roughly several hundred seconds.
In our study, we measured the Rxx much faster than the decay time. We changed the current to a suﬃciently
small value 5 nA and measured the temperature dependence of RES within 50 s after DNP at a large current of 60
nA for 2,000 s. Figure 1 shows the temperature T dependence of Rxx of after DNP by AC and DC current and before DNP. Surprisingly, we observed an unexpected phenomenon that the derivative of Rxx with respect to T is
negative after AC current pumping [3]. This result indicates that the enhanced resistance state is an insulatingphase transitioned from the QHS. Then, we measured the pump current frequency fpump dependence of RES. Inset
of Fig. 2 shows the time evolutions of Rxx with several fpump. In Fig. 2, fpump dependence of Rxx after 3,600 s pumping are illustrated. Obviously the resistance enhancement effect is large in high fpump and weaken with decreasing
fpump. DC pumping causes the small resistance state (~5 kΩ) which has positive derivative of Rxx with respect to T
(Fig. 1). This result suggests that the oscillation of the electric field is an important role for the Rxx enhancement.
In this study, we first found that the RES is an insulating phase transitioned from the QHS by AC current. This
new discoveries shed new light on the nuclear polarization mechanism in the ν = 2/3 QHS.
3530

Rxx (kΩ)25
20
15

after DNP (AC)before DNP
after DNP (DC)

10
5
0

100

150

200

250

300

T (mK)

Fig.1. The temperature dependence of Rxx
Fig.2. The pump current frequency fpump dependence of RES.
after DNP and before DNP.
References[1] S. Kraus, O.Stern, J. G. Lok, W. Dietsche, K. von Klitzing et al., Phys. Rev. Lett. 89 (2002) 266801.
[2] S. Tsuda, M. H. Nguyen, A. Fukuda, Y. Zheng, T. Arai and A. Sawada, 31st ICPS proceedings (2012).
[3] S. Tsuda, M. H. Nguyen, D. Terasawa, A. Fukuda and A. Sawada, submitted to J. Phys. Soc. Jpn.

Novel mesophase in the region between Cholesteric andPerforated Lamellar -Nematic phases
Softmatter physics group, Jun Yoshioka
Abstract We experimentally analyzed the system where fluorinated amphiphiles are dispersed in thecholesteric solvent using the measurements of the layer compression, the translational diffusion, and theviscosity constants. In summary, we found a novel mesophase (Ch2) appears between the cholestericand the perforated lamellar-nematic phases.
© 2013 Department of Physics, Kyoto University

Introduction
Recently, we found the perforated lamellar-nematic (PLN) phase appears in the system where the amphiphilicmolecules (BI) having both a hydrocarbon and a fluorocarbon chain are dispersed in the thermotropic nematicsolvent. In the PLN phase, the perforated bilayer structure is embedded in the nematic solvent. In this paper,by introducing the chirality into the nematic solvent, we realized the system where the BI molecules are dispersedin the cholesteric (Ch) liquid crystal, and found the novel mesophase (Ch2) appears between the Ch and the PLN
phases. We also discussed the physical properties of the novel Ch2 phase using several experiments.

Layer compression modulus [106 N/m2]

temperature [deg]Results and Discussion
First, using the polarized microscopy, we made a concentration-temperature phase diagram of the mixture asshown in Fig.1. The Ch phase appears when the concentration of BI is low (0~30wt. %), while the PLN phaseappears when the concentration of BI is high (35~50wt. %). In the sandwich cell where the helical axis is forced
to be parallel to the glass substrates, we found the texture of the sample significantly changes dependent on thetemperature in 28~35 wt. % BI mixture. Here, we
100
speculated a novel mesophase appears in the region between
90
the Ch and the PLN phase (hatched region), and call thisIsotropic(I)80
new mesophase as the Ch2 phase.
I+PLN
Second, in order to confirm the existence of the Ch2 phase,70
we measured temperature dependence of the layer
60
Ch2+PLN
compression modulus as shown in Fig.2. The Ch phase has
Ch
Ch2
PLN
50
a pseudo-layer structure derived from its helix, so that it has
a non-zero layer compression modulus along the helical axis.
40
0
10
20
30
40
50
In the Ch phase, the layer compression modulus shows3
4
2
concentration of BI [wt%]10 –10 N/m , which is the typical value in the conventional
Fig. 1. Phase diagram
Ch phase. The modulus starts to increase just below the
Ch-Ch2 transition temperature, and it attains to 106N/m2 in
1
the Ch2 phase at 40°C. This result shows the Ch2 phasehas a much stiffer layer structure.
Furthermore, we also detected the Ch-Ch2 transition by
0.1
the translational diffusion constants and the viscosity
Ch2
Ch
modulus measurements. Thus, the Ch2 phase can be
identified as a novel mesophase due to the frustration
0.01
between the layer order of the aggregates of the BI molecules
and the helical structure of the solvent.
35

40

45

50

55

60

65

70

temperature [deg]Fig. 2. Temperature dependence of layer
compression modulus in 30wt. % BI mixture